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Za is caleulated from the horizontal plane temporalward downward. 
In Fig. 1 OP lies consequently in the third quadrant, in Fig. 2 in 
the fourth quadrant. [f now we call OF positive, when RF lies tem- 
porally and PR positive when F lies under the horizontal plane, 
then two values will in general satisfy the formula for sin a, 
and also two that for tg ¢, but only one value will satisfy both for- 
mulas ; consequently «a is determined. 
For the first part of the investigation we have consequently cal- 
culated the angles A, D, H and « from the data a, PR and OR. 
In the second part of the investigation we must learn to know 
the rotation of the eye round its line of regard. If we call the 
position that the eye must assume according to the law of Listing 
during its different directions of regard the normal position. then we 
can make it our task to ascertain, if, how much and in what direction 
the eye has rotated round its line of regard in relation to that 
normal position. . 
Let ( again be the point of fixation and P the point where the 
line of regard of the examined eye cuts the wall. Now we place a 
source of light in fand hold before the examined eye again the 
rods of Mappox, as perpendicularly to the line of regard as possible. 
We now move the rods in their plane so long, till the red line is 
apparently seen horizontal (as the thin dotted line in Fig. 3 going 
+ 
through (QQ indicates). Leaving the rods in this position we remove 
the flame from P and move it in a circle round P. The points 
where the flame is, when the red line goes again apparently through 
(2 (namely P' and P") are marked, and it needs no further demon- 
